Alzheimer's Disease
Alzheimer's Disease (AD) is the most frequent cause of dementia in the United States, affecting over two million individuals each year. It is a degenerative brain disorder characterized clinically by loss of memory, confusion, and gradual physical deterioration. It is the fourth most common cause of death. The etiology of the disease is virtually unknown but has been attributed to various viruses, toxins, heavy metals, as well as genetic defects. The disease is at present incurable.
Until quite recently, AD was thought to account for relatively few of the cases generally classified as senile dementia. Other factors can lead to such a condition, including repetitious mild strokes, thyroid disorders, alcoholism, and deficiencies of certain vitamins, many of which are potentially treatable. It can be appreciated, then, that a diagnostic test specific for AD would be very useful for the clinical diagnosis and proper clinical treatment of subjects presenting with symptoms common to all of these conditions.
The brains of individuals with AD exhibit characteristic pathological accumulations of congophilic fibrous material which occurs as neurofibrillary tangles within neuronal cell bodies, and neuritic (or senile) plaques. Neurofibrillary tangles may also be found in the walls of certain cerebral blood vessels. The major organized structural components of neurofibrillary tangles are paired helical filaments. Qualitatively indistinguishable amyloid deposits also occur in normal aged brains but in much smaller numbers with restricted topographical distribution.
There has been considerable recent investigative activity regarding the characterization of proteins found in neuritic plaques and neurofibrillary tangles of AD and other neurologic diseases. One of the amyloid proteins initially described by Glenner et al. has been cloned and sequenced (Glenner et al., Biochem. Biophys. Res. Commun. 120:1131-1135 (1984); U.S. Pat. No. 4,666,829). The A4 amyloid protein found in neuritic plaques and blood vessels has been determined to be a component of a 695 amino acid precursor; a protein postulated to function as a glycosylated cell surface receptor (Masters et al., Proc. Natl. Acad. Sci. USA 82:4245-4249 (1985), Kang et al., Nature 325:733-736 (1987)). In addition, the amyloid protein has been postulated to function as a cell adhesion molecule and as a calcium ion channel protein (Hooper, J. NIH Res. 4: 48-54 (1992); Rensberger, Wayward Protein Molecule May Be Elusive Killer of Brain Cells, The Washington Post, Jan. 25, 1993, .sctn.1, at A3 (1993)). The gene coding for A4 is located on chromosome 21 (Kang et al., ibid.; Goldgaber et al., Science 235:877-880 (1987); Tanzi et al., Science 235:880-885 (1987); St. George-Hyslop et al., Science 235:885-889 (1987)) but apparently is not linked to the familial form of the disease (Van Broekhoven et al., Nature 329:153-155 (1987)). There appears to be little, if any, protein sequence homology between amyloid A4 and .beta. protein, their higher molecular weight precursor, and pancreatic thread protein (PTP) (Gross et al., J. Clin. Invest. 76:2115-2126 (1985)).
A number of other proteins thought to be associated with the disease have been described, including Ubiquitin, ALZ-50, microtubular-associated proteins .tau. and MAP2, and neurofilament protein (see, for example, Manetto et al., Proc. Natl. Acad. Sci. USA 85:4502-4505 (1988); Wolozin et al., Science 232:648-651 (1986); Selkoe, Neurobiol. Aging 7:425-432 (1986); Perry et al., in: Alterations of the Neuronal Cytoskeleton in Alzheimer's Disease, Plenum, New York, pp 137-149 (1987)). More recently, a serine protease inhibitor called .alpha..sub.1 -anti-chymotrypsin has been found in AD amyloid deposits (Abraham et al., Cell 52:487-501 (1988)).
There is currently no useful diagnostic test for AD being practiced clinically. A definitive diagnosis is possible only postmortem, or during life through a brain biopsy, to reveal the presence of the characteristic plaques, tangles, paired helical filaments, and other cerebrovascular deposits which characterize the disorder. Such an invasive surgical procedure is inherently dangerous and is therefore rarely utilized. As a result, the clinical misdiagnosis of AD is estimated to be approximately 20%-30%.
Thread Proteins
The prototype thread protein molecule is pancreatic thread protein (PTP) which bears the unusual physical property of forming insoluble fibrils at neutral pH, but is highly soluble at acid or alkaline pH (Gross et al., supra). PTP is highly abundant, synthesized by pancreatic acinar cells, and secreted into pancreatic juice in concentrations exceeding 1 mg/ml (Id.). An increased thread protein immunoreactivity has been demonstrated in brains with AD lesions, using monoclonal antibodies to PTP (Ozturk et al., Proc. Natl. Acad. Sci. USA 86:419-423 (1989)). In addition, a highly sensitive forward sandwich immunoradiometric assay was used to demonstrate that at least three distinct antigenic epitopes were shared between PTP and the related protein in the brain (Id.) Despite similarities, the pancreatic and neuronal forms of the thread protein are almost certainly distinct since the mRNA molecules and proteins differ in size, and many of the antigenic epitopes which are present in the pancreatic thread protein are not detectable in brain tissue (de la Monte et. al., J. Clin. Invest. 86:1004-1013 (1990); de la Monte et. al., J. Neurol. Sci. 113:152-164 (1992); de la Monte et al., Ann. Neurol. 32:733-742 (1992)).
The central nervous system form of the thread protein, designated hereafter as "neural thread protein" (NTP), has been identified in AD and Down's Syndrome brain tissue (Wands et al., International Application Publication No. WO 90/06993). NTP has been found in all AD brains studied where characteristic neuropathologic changes of the disease exist (Id.). The saline-extractable soluble immunoreactivity shares has a molecular weight of approximately 17 to 20 kD (Id.).
Quantitative measurements of NTP immunoreactivity in various regions of AD brains revealed levels varying from 12 to 295 ng/gm tissue (Mean=116 ng/gm tissue) compared to 1-11 ng/gm tissue (Mean=5 ng/gm tissue) in comparable ares of control brains (Id.).
Immunocytochemistry performed with monoclonal antibodies directed against the pancreatic form of PTP demonstrated that NTP is localized within cells, within fine processes within the neuropil, or is extracellular in both AD and Down's Syndrome brains (Id.). Two types of cell contain NTP: neurons and astrocytes (Id.). The affected neurons are the large pyramidal type which typically contain the neurofibrillary tangles well known in AD brain (Id.).
That NTP accumulation within neurons is intrinsically important or integrally related to the evolution of AD lesions is corroborated by the presence of identical patterns of immunolabeling for NTP in Down's Syndrome brains, but not in control brains (Id.). It is important to note that the same structural abnormalities of AD occur in brains of all middle-age individuals with Down's syndrome, whether or not they are demented. There is also a higher incidence of AD in family members of Down's Syndrome patients. Moreover, the regional differences in the densities of NTP-containing neurons parallels the density distributions of neurofibrillary tangles in both AD and Down's Syndrome. This provides further evidence that NTP is germane to the pathophysiology of AD. Whether NTP accumulates within neuronal perikarya, as a result of aberrant cellular metabolism or transport is not yet known.